Highlight missing <endif>

[Clashsoft]

Since there is no completion (#20) yet, it is easy to miss an <endif>. As such, it would be very useful if this fact were highlighted with an error indicator.

At runtime, this problem is reported as premature EOF at a seemingly random line in the file, which is very hard to figure out.

[Clashsoft]

As of 0.7, it now looks like this:

The error message is not perfectly clear but it's better than nothing.

[bjansen]

I'm not sure how we could tell the parser to report a better error message. I believe the message is intentionally "vague" because at this point there could be <elseif>, <else> or <endif>, all of which start with the LDELIM token. @parrt what do you think?

[parrt]

Yes, the parser only knows that it has to have a tag to go with that and, as it likely is missing < as a token (not a char), probalby best we can do

[Clashsoft]

Alright then, if it's too much work. I'm personally already happy with that error message.